Fundamentals of Transportation/Metering

Ramp metering works by keeping traffic just below capacity, at the sweet spot of freeflow speed and maximum throughput

Metering (or Ramp Metering) is the application of traffic control to freeway on-ramps to limit the rate of flow entering freeway sections.

Metering has several purposes. The dominant purpose is optimize traffic flow. Metering keeps traffic flowing at or near freeflow speed and at or near maximum flow. Since is its just below the maximum flow, it can be thought of as risk averse. How far below the maximum you set the metered flow rate is an indication of risk aversion (how important it is to avoid breakdown. However, by maximizing total output flow, while avoiding throughput-reducing traffic breakdowns, metering flow should also maximize traffic flow on other facilities as well.

Metering has other objectives, including the break-up of platoons entering freeways. By limiting the number of vehicles that enter the freeway at once, the likelihood of each vehicle finding a suitable gap into which it can merge is increased, and thus the likelihood of generating shockwaves at freeway on-ramps decreased.

Metering similarly improves safety by reducing the number of stop-and-start maneuvers required, and decreasing the likelihood of crashes.

Finally, metering provides a tool to manage incident conditions dynamically. Incidents may block lanes, and metering restricts inflow so that congestion on the freeway is diminished. If ramp queues are long enough, vehicles may divert to other routes.

Metering does not come for free, it imposes costs, which are increased delays on ramps. When determining the appropriate metering rate, ramp delays created must be considered along with freeway delays reduced.

The absence of metering resulted in increased freeway system (freeway plus ramp) travel time (reduced speeds) for most travelers. However short trips were advantaged as they did not need to wait at ramp meters to use the freeway system. Travel time variability on the freeway system increased, as freeway congestion was more frequent (though ramp delays were now nonexistent). However the equity of the system increased, as delays were more uniformly distributed.

The number of trips using the freeway system tended to increase. Average weekday peak period trip lengths on the freeway system dropped. We can infer that more non-work trips used the freeway during this period.

It turns out, the most efficient ramp metering control logic is the one metering the nearest entrance ramp(s) to a critical freeway section so as to keep the flow of this section strictly below capacity. This is also the least equitable, as it implies travelers from one ramp may receive all of the delay while others don't.

In determining a control strategy, equity and efficiency are traded-off. There are a number of implementation considerations:

Control – how to “keep the flow of this section strictly below capacity”.

Control variables: Flow vs. Density,

Feedback vs. Feed-forward,

Linear vs. Nonlinear

Selection of the threshold values for the controller

Reliability: Risk averse vs. Risk seeking

Equity considerations – Can one only meter the “nearest entrance ramp(s)”